Planar Lightwave Circuits (PLCs) transmit and receive signals for both short distance data and long distance telecommunication systems. For optimal operation, the PLCs have functional optical components, such as waveguides. These waveguides should be small enough in size so that dense integration of waveguide optical devices, including sharp bends in the waveguides, is possible on a single chip.
High-index-contrast material systems work well for fabricating PLCs. Such material systems can be formed as a core layer of silicon, on a Silicon On Insulator (SOI) substrate, The core layer has a refractive index of about 3.5, and it is surrounded on both side by silica cladding with a refractive index of about 1.5. A waveguide formed in such a structure is called a channel waveguide. Channel waveguides offer better light confinement in smaller dimensions.
There are potentially many practical uses of high-index-contrast waveguide chips, especially in telecommunications, where there is a need for developing ways to route and process multi-wavelength optical signals transparently (i.e., without having to convert optical signals to electrical signals for processing, and then back again). One example of a PLC waveguide chip is a single-mode waveguide-based “mux” and “demux” for serializing and separating multi-wavelength optical signals in Dense Wavelength Division Multiplexing (DWDM) applications. For this kind of application and others, it is generally desirable to configure the waveguides so as to force single-mode propagation to avoid introduction of undesirable effects of differing propagation velocities of different optical modes.
One of the most difficult challenges facing high-index-contrast optical chips is efficiently coupling light into and out of the chip. Particularly difficult is the coupling of light from a standard optical fiber or external source to a silicon waveguide on the chip. A large mismatch between the common optical fiber dimensions and that of the high-index-contrast waveguide, and their respective mode sizes, impairs light coupling from optical fibers to PLC waveguides.
A number of techniques have been utilized for optical coupling between waveguides and optical fibers, including prism couplers, grating couplers, tapered fibers and micro-lens mode transformers. Unfortunately, these techniques do not offer the combination of high coupling efficiency, wavelength independence, reliability, manufacturability, ruggedness, and robustness demanded for use in low-cost, high-volume telecommunications applications.
In view of the foregoing, there is a need for an improved coupling between optical fibers and planar waveguides formed on optical chips, such as PLCs.